Composite materials comprising PPTA and nanotubes

ABSTRACT

Disclosed is a composite material comprising PPTA (poly-p-phenyleneterephthalamide) and nanotubes having an aspect ratio of at least 100 and a cross-sectional diameter of 5 nm or less, the composite material containing up to 12 wt. % of nanotubes, obtainable by adding the nanotubes to sulfuric acid, decreasing the temperature to solidify the mixture, adding PPTA to the solid mixture, heating to above the solidifying point and mixing the mixture, and spinning, casting, or molding the mixture to the composite material.

This is a Continuation of application Ser. No. 10/588,160 filed Aug. 1,2006, now U.S. Pat. No. 7,534,486, which is a National Phase ofApplication No. PCT/EP2005/002931 filed Mar. 18, 2005, which claimspriority to EPO 04006765.4 filed Mar. 20, 2004. The disclosures of theprior applications are hereby incorporated by reference herein in theirentireties.

The invention pertains to composite materials comprising PPTA(poly-p-phenyleneterephthalamide) and nanotubes, to a spin dope solutioncomprising the same, and to a process of making said solution, and tomultifilament fibers made thereof.

Composites of single-wall carbon nanotubes (SWNT) and aromaticpolyamides are known from WO 03/085049. According to this referencearomatic polyamide is added to SWNT to form the composite. It was alsodisclosed that aromatic polyamide can be mixed with SWNT in an acid toform a dope, which dope can be spun into a fiber or film. Thehomogeneous dope mixture was obtained by mixing SWNT and PPTA insulfuric acid at 80-85° C. for several hours. The preferred aromaticpolymer is PPTA. The method used in this reference has variousdisadvantages. For instance, if fibers are made only monofilament fibersare obtained. Further, tensile strength and modulus are relatively low.Tensile strength of 0.33 to 0.35 GPa and modulus of 13 to 19 GPa wereobtained with the as-spun fiber. A further disadvantage of this methodis the need of large quantities of SWNT in the mixture. According tothis reference about 5 to 10 wt. % of SWNT, based on the total weight ofSWNT and PPTA, is necessary to obtain composite materials with the abovetensile strength and modulus. Because SNWT are extremely expensivecompounds, this is a serious burden to commercialization of suchproducts.

In EP 1336673 a method is disclosed for producing composite materialscomprising carbon nanotubes. The composite materials arepolyethylene-base materials, but generically it has been disclosed thatother polymers such as PPTA can also be used. It was now found that theprocess as disclosed in this reference does not lead to compositeproducts according to the present invention, i.e. products that can bespun, when PPTA rather than polyethylene is used. This is furtherdemonstrated in Comparison examples 4 and 5.

In WO 03/080513 a composition is disclosed comprising a highly dispersedmixture of a polymeric material and nanostructures in a liquid medium.The polymeric material is selected from a large group of polymers,including aromatic polyamides. The specific examples of this referencedescribe composite materials made from epoxy resin exclusively. It wasnow found, as demonstrated in Comparison examples 4 and 5 that a processas disclosed in this reference does not lead to composite productsaccording to the present invention, i.e. products that can be spun, whena highly dispersed mixture of PPTA and nanostructures (SWNT) in a liquidmedium (sulfuric acid) is used.

There is a need for composite materials of nanotubes and aromaticpolyamides with higher tensile strength and higher modulus, obtainedfrom a spin dope that is also suitable for making multifilament fibersand yarns, and which may contain small quantities of nanotubes withoutdetrimental loss of tensile strength and modulus. The compositematerials should further have excellent compression strength, andpreferably have flame-retardant properties.

It is an object of the invention to obtain a substantial improvement ofthe known method for making composite materials comprising nanotubes andaromatic polyamides. To this end the invention relates to a compositematerial comprising PPTA (poly-p-phenyleneterephthalamide) and nanotubeshaving an aspect ratio of at least 100 and a cross-sectional diameter of5 nm or less, the composite material containing up to 12 wt. % ofnanotubes (based on the total weight of nanotubes and PPTA), obtainableby adding the nanotubes to sulfuric acid, decreasing the temperature tosolidify the mixture, adding PPTA to the solid mixture, heating to abovethe solidifying point and mixing the mixture, and spinning, casting, ormolding the mixture to the composite material.

More particularly, the invention pertains to a method for making a spindope solution comprising the steps:

-   a) adding the nanotubes having an aspect ratio of at least 100 and a    cross-sectional diameter of 5 nm or less to sulfuric acid at a    temperature above the solidifying point of the sulfuric acid;-   b) decreasing the temperature to below the solidifying point of the    sulfuric acid and mixing for a sufficient time to solidify the    mixture;-   c) adding PPTA to the solid mixture; and-   d) heating to above the solidifying point and mixing the mixture.

By the solidifying point of concentrated sulfuric acid is to beunderstood within the scope of the invention the temperature at whichfor the first time a solid phase begins to form in the liquid sulfuricacid which is cooled with stirring. Values for the solidifying point ofconcentrated sulfuric acid can be found in the literature. The term“concentrated sulfuric acid” means sulfuric acid having a strength of atleast 96% by weight. Use may be made of concentrated sulfuric acidcontaining up to 20% by weight of free SO₃. The sulfuric acid to be usedin step b) of the process according to the invention may have anytemperature below its solidifying point. Considering that there areeconomical and technical disadvantages to the use of extremely lowtemperatures, however, the temperature to be chosen will generally notbe more than 50° C. below the solidifying point of the sulfuric acid tobe used. The temperature of the sulfuric acid, cooled down to below itssolidifying point, is preferably lower than 0° C. Moreover, to preventpremature melting of the solid sulfuric acid it is preferred that useshould be made of sulfuric acid cooled down to at least 5° C. below itssolidifying point. The temperature of the PPTA to be combined with thesulfuric acid may be equal to, or higher or lower than room temperature,but need be so chosen that during the adding and the mixing processesthe mixture remains in the solid state. Extremely high temperatures ofthe PPTA to be combined with the sulfuric acid will therefore beavoided. To prevent any heat introduced into the system by the PPTA orevolved in the mixing process from prematurely causing the mixture tomelt it may be necessary to apply cooling during the process of bringingtogether the mixture of sulfuric acid and nanotubes and the aromaticpolyamide and the mixing thereof. The temperature should preferablyremain below the solidifying point of the sulfuric acid until themixture has reached the homogeneity required for it to be used asspinning mass. If desired, the PPTA before it is combined with thesulfuric acid, may be cooled to below room temperature, for instance tobelow the solidifying temperature of the sulfuric acid. The preparationof the sulfuric acid cooled to below its solidifying point may beeffected in various ways. The procedure is preferably such that thesulfuric acid is brought into a finely divided state before it iscombined with the aromatic polyamide, which is also in a finely dividedstate, and mixed. By a finely divided state is to be understood withinthe scope of the invention a mass made up of particles whichindividually measure less than about 2 mm and preferably less than about0.5 mm. Such particles may be bonded together to form conglomerateswhich during mixing are again split up into separate particles.Particularly, the finely divided sulfuric acid may be present in a statewhich strongly resembles that of snow. The sulfuric acid should alwaysbe so finely divided that upon mixing it with PPTA it forms a mixturesuitable to be used as spinning mass.

To obtain the advantages of the process according to the invention it isnecessary but not sufficient that concentrated sulfuricacid/nanotube/PPTA mixture should be intermixed at a temperature belowthe solidifying point of the sulfuric acid. Essential to the inventionwhen making a spinning mass is that the concentrated sulfuric acid iscooled down to below its solidifying point before it comes into contactwith the aromatic polyamide. Bringing together liquid sulfuric acidhaving a temperature above its solidifying point and finely divided PPTAfollowed by stirring at low shear rate conditions at a temperature belowthe solidifying point of the sulfuric acid will generally result inobtaining a non-homogeneous mixture which is not or hardly suitable forspinning purposes.

Surprisingly, the solidification step substantially improves the tensilestrength and modulus of composites made of the above spin dope and makeit possible to use minor amounts of nanotubes. Composite materials thancan be made are, inter alia, fibers and films. The invention thereforealso has the object of obtaining a fiber, particularly a multifilamentfiber obtainable from the hereinabove mentioned spin dope solution. Moreparticularly, the multifilament fiber preferably contains at least 5filaments, more preferably at least 20 filaments.

A similar method of applying a solidification step in making mixtures ofsulfuric acid, PPTA, and inorganic whiskers is known from U.S. Pat. No.5,512,368. Thus according to Example 1 of that reference silicon carbidewhiskers were added to and mixed with concentrated sulfuric acid, afterwhich the mixture was frozen and PPTA was added. This mixture can beused as a spin dope for making monofilament microcomposite fiber. Thewhiskers used in this method, however, are not comparable with thenanotubes of the present invention. Thus the whiskers according to thisreference are inorganic materials, particularly silicon carbide orsilica, having a preferred aspect ratio of 5 to 50 and a cross-sectionaldimension of about 0.1 to 1.5 μm at an average length of about 2 to 20μm. These whiskers thus have dimensions that are a magnitude greaterthan the instantly claimed carbon nanotubes, and due to their giantdimensions are generally contained in high quantities in the fibermaterial, such as 25 wt. % according to Example 1.

According to the invention nanotubes are molecules made purely ofcarbon. Examples of such molecules are the buckyball orBuckminsterfullerene (C60=sixty carbon atoms in a spherical shape). Suchmolecules can, however, be modified such as by Diels-Alder reaction withunsaturated molecules having functional groups such as hydroxy, amino,and carboxyl groups. Such modified nanotubes are also encompassed withinthe protection sought. The term “nanotubes” particularly refers totubular molecules such as tubular fullerenes, which is a tube that maybe capped at each end by two hemispheres of C60 with only hexagonaland/or pentagonal units in its side walls. Further multi-walled carbonnanotubes (MWNTs: concentric cylinders of carbon) such as formed in acarbon arc discharge process are within the present definition ofnanotubes. Preferably, however, the nanotubes are single-walled carbonnanotubes (SWNTs). Single-walled carbon nanotubes are tubes in which asingle layer of graphite (graphene) is rolled up into a tube. Grapheneconsists of carbon atoms in a hexagonal structure like chicken wire. Therolling up can be accomplished in various ways. For example,carbon-carbon bonds can be parallel or perpendicular to the tube axis.Alternatively, the carbon-carbon bonds can be directed between paralleland perpendicular to the axis. The differently wrapped tubes aredistinguished from one another by a double index (n,m), where n and mare integers. This double index specifies the number of unit vectors(a₁, and a₂) required to connect two atoms in the planar hexagonallattice to form a tube.

The main impurities in SWNT material resulting from the differentproduction processes are multi shell carbon nanocapsules (“bucky onions”that are produced to deactivate large catalyst particles), which areempty or filled with transition metal, amorphous carbon nanoparticles,MWNTs, uncapped catalyst particles, substrate particles (e.g. SiO₂),graphite and fullerenes. These impurities can be removed before SWNTscan be utilized in a composite material. Highly pure SWNT material,which comprises at least about 85% SWNTs, is preferred over lower puritymaterial. For example in WO 98/39250 the SWNTs are heated underoxidizing conditions to remove amorphous carbon and other contaminatingmaterials. The SWNTs are refluxed (at 120° C.) in an aqueous solution ofan oxidizing agent (e.g. HNO₃, a mixture of H₂O₂ and H₂SO₄, or KMnO₄) ata concentration which is sufficiently high to etch amorphous carbon butnot too high to prevent that the SWNTs will be etched. Usefulconcentrations are preferably in the range 2.0-2.6 M nitric acid.

The nanotubes according to this invention have an aspect ratio of atleast 100 and a cross-sectional diameter of 5 nm or less. Preferably,the nanotubes have aspect ratios greater than 150, more preferablygreater than 200, and a cross-sectional diameter of less than about 2nm.

SWNTs are important nanotubes according to the invention because theycan reinforce PPTA fibers, for instance by incorporation at void regionsor by bridging between two crystalline domains. In void regions the SWNTshould interact with the polymer chains in the crystal. This can beaccomplished by Van der Waals interactions. However, it may alsopossible to modify the surface of SWNTs in such a way that the hydrogenbonds are formed between a group on the SWNT surface and the amidegroups that do not participate in the hydrogen bonds in the crystal(approximately one third of the amide bonds in aromatic polyamides, suchas Twaron®). In order to act as a bridge between the crystalline regionsin a fibril, the SWNT should propagate through various crystallineregions. Consequently, it is preferred that the SWNT has a length of atleast 100 nm (bridging three crystalline regions).

It is preferred that the nanotubes possess no curvature in the directionof the fiber axis. Curvature can dramatically reduce the mechanicalproperties of the nanotube/polymer composite. Although it is expectedthat nanotubes are aligned in the fiber direction through the liquidcrystalline phase and drawing, special attention should be paid to thecurvature. The best results are expected when the nanotubes areperfectly aligned along the fiber axis.

The term PPTA as used in the present invention stands forpoly-p-phenyleneterephthalamide, which polymer is made by polymerizingpara-phenylenediamine (PPD) as aromatic diamine monomer andterephthaloyl dichloride (TDC) as para-oriented aromatic dicarboxylicacid halide monomer. The definition of PPTA throughout this inventionalso includes such polymers wherein small quantities (less than 10 mole%, preferably less than 5 mole %, most preferably less than 2 mole %) ofPPD and/or TDC are replaced by other aromatic diamine or dicarboxylicacid halide monomers, such as 2,6-naphthalenedicarboxylic aciddichloride, 2-chloroterephthaloyl dichloride, isophthaloyl dichloride,and 2,5-diamino-benzenesulfonic acid.

Preferably, the mixture is mixed in step a) for 10 min to 6 h at 10 to90° C., more preferably 30 min to 4 h at room temperature to 70° C.,most preferably at 45-55° C. The nanotube before addition is preferablyfirst dried, preferably at elevated temperature (for instance about 80°C.) at vacuo for 2 to 24 h. Most preferably, the nanotubes are welldispersed in sulfuric acid (homogeneously distributed individualnanotubes) before ice making. It is preferred to disperse the nanotubesby a sonification process to improve and to speed up the formation ofthe dispersion. Sonification can be performed with the usualsonification apparatuses, generally by sonification for 10 minutes to 24hours at 10 to 90° C., for instance for 3 hours at room temperature.Once the solution is transformed into ice, by decreasing the temperatureto below the solidifying point of the sulfuric acid, generally to 7 to−20° C., preferably to 2 to −12° C., it can be mixed with PPTA to form asolid spinning solution. Prior to adding PPTA to the mixture thetemperature is preferably maintained at −5 to 0° C.

The well dispersed nanotubes in sulfuric acid can penetrate into theporous PPTA structure. Mixing is very critical in order to obtain a gooddispersion of the nanotubes in the PPTA spinning solution. The mixtureis mixed for at least 1 h before increasing the temperature and then thetemperature is preferably elevated to ambient temperature under mixing.

According to the process of the invention use is made of a mass which isprepared by intermixing PPTA and concentrated sulfuric acid/nanotubemixture in the solid state. Preferably, not until the sulfuric acid andthe aromatic polyamide have completely been intermixed to a homogeneousmixture is the temperature of the mixture allowed gradually to rise toabove the solidifying point of the sulfuric acid used. Although meltingof the solid sulfuric acid particles might then be expected to give riseto the formation of a liquid sulfuric acid phase, such a phase is notnoticeable in actual practice. In spite of the present mixtures ofconcentrated sulfuric acid/nanotubes and PPTA generally consisting of 75to 85% by weight of concentrated sulfuric acid, they have, even attemperatures above the solidifying point of the sulfuric acid used, forinstance above room temperature, a dry and sandy character. Apparentlythe sulfuric acid present is entirely absorbed by the polymer particles.For such a mixture to be spun it must, of course, be heated to a highertemperature. Depending on the composition of the polymer, theconcentration and the inherent viscosity of the polymer, the temperaturewould have to be in the range of 20° C. to 120° C.

Bringing the sulfuric acid/nanotubes and the aromatic polyamide togetherin step c) may be effected in various ways. The sulfuric acid/nanotubesmay be added to the aromatic polyamide or inversely. It is also possiblefor the substances simultaneously to be brought into a suitable space.The continuous preparation of the spinning mass may be carried out forinstance with the aid of a mixer consisting of a housing provided withcooling elements and a rotary screw. Liquid sulfuric acid or a mixtureof liquid sulfuric acid and nanotubes is fed into the inlet side of thehousing, in which it is cooled. Into a following section, there wherethe temperature of the sulfuric acid has sufficiently decreased, thefinely divided aromatic polyamide is added. The rotary screw will thenalso serve as mixing device. Then the solid mixture has reached thedischarge side of the housing, it is sufficiently homogeneous to be usedas spinning mass. Particularly suitable is the method by which into avessel provided with a cooling device and a stirrer a liquid,concentrated sulfuric acid is introduced and subsequently converted,with stirring and cooling, into a snow like mass and subsequently, withcontinued stirring, the finely divided aromatic polyamide mixture isadded.

The temperature of the PPTA/nanotubes/sulfuric acid mixture is elevatedto above the solidifying point in step d) by applying conventionalheating means.

The composite material obtained has a tensile strength of at least 1.5GPa, preferably at least 2 GPa, and a modulus of at least 50 GPa,preferably at least 70 GPa, and can be made of compositions containing12% by weight or less, preferably about 5% by weight, most preferablyabout 1% by weight based on the total of nanotubes and PPTA.

The compositions of the invention, particularly the films and fibersmade of the spinning solutions are suitable for applications where hightenacity, high modulus, and high compression strength are of importance,such in composites for automotives, bullet resistant materials,including soft and hard ballistics.

The following is an experimental example of the invention, which isillustrative of the inventions and should not be interpreted aslimitative.

EXAMPLE 1

A spinning solution (dope) was produced in a Drais mixer (type FH6) of 6liter. During all spinning solution preparation steps, nitrogen waspurged into the Drais mixer. The mixing chamber of the Drais mixer is adouble-walled chamber. The following procedure was used to prepare thespinning solution.

-   -   Adding 2001 grams of sulfuric acid (99.8% by weight) to a        preheated mixing chamber (wall temperature=50° C.) while purging        the system with nitrogen.    -   Heating the mixture to a temperature of 50° C.    -   Adding 4.94 grams of single walled carbon nanotubes (SWeNT™ dry        85% purified SWNTs (lyophilized) grade S-P95-dry; SouthWest        Nanotechnologies, Norman, USA) to the sulfuric acid. The SWNTs        were dried at 80° C. under vacuum for 8-10 hours.    -   Mixing the solution (liquid sulfuric acid and the SWNTs) (mixing        speed=20 rpm) for 120 minutes at a temperature of 50° C.    -   Transferring the mixture to a plastic storage bottle and        sonicating (Bandelin, Sonorex super RK 1028H, 35 kHz) mixture        for 3 hours (no heating was applied).    -   Transferring the mixture to the Drais mixer and decreasing the        temperature to −10° C. (temperature of mixing chamber wall        resulting in a mixture temperature of approximately −2° C.) and        waiting for 2 hours. The sulfuric acid became solid.    -   Increasing the temperature to −2° C. (temperature of mixing        chamber wall), adding 489 g of PPTA (having a relative viscosity        5.1) to solid mixture after 45 minutes and mixing for 1 hour at        a wall temperature of −2° C.    -   Cooling was stopped, and mixing was performed for 11 hours. The        temperature of the mixture slowly increased to ambient        temperature.

EXAMPLE 2 Functionalization of Purified SWNT

The SWNTs were purified according to the following procedure:

3.11 g of SWNT (ex Nanoledge) were moistened in a blender (Waringcommercial blender) in 300 ml of deionized water. To obtain sufficientmoistening the suspension was evacuated three times in vacuo in an oven(room temperature, pressure smaller than 50 mbar) for 15 minutes. TheSWNTs were settled after 12 h. The supernatant was sucked off and thepaste was transferred to centrifuge tubes and centrifuged for 30 min at4000 rpm (Heraeus, Megafuge 1.0). The resulting paste, 178.6 g ofdeionized water, and 58.17 g of HNO₃ (65%) were brought into a highpressure-resistant beaker, and heated in a microwave oven (MilestoneMicrosynth) to 180° C. (10 bar). The temperature of 180° C. was reachedwithin 5 min (1000 W) and this temperature was kept for 1 h (80 W). Thecooled mixture was centrifuged several times (4000 rpm; 30 min) usingdeionized water until the supernatant had reached pH˜7.

The paste was suspended in about 300 g of NMP (N-methylpyrrolidone),stirred and centrifuged for 30 min at 2000 rpm. The supernatant wassucked off and this procedure was repeated four times. The paste wascentrifuged for another three times with 300 g of IPA (isopropylalcohol). The paste was dried for 24 h in a vacuum oven at 160° C. (10⁻¹mbar).

The reaction between 4-amino-phenyl citraconimide (APCI) or4-amino-phenyl maleimide (APMI) and purified SWNT was performed asfollows:

A reaction tube containing 0.10 g of above purified SWNT and 0.20 g ofAPMI, and a tube containing 0.11 g of above purified SWNT and 0.21 g ofAPCI in 100 g of NMP were evacuated in a vacuum oven at roomtemperature, followed by an ultrasonic treatment (Bandelin, SonorexDigital 10P) of 30 min at room temperature, and evacuated again. Thereaction tubes, provided with a magnetic stirrer, were heated at 140° C.for 24 h under nitrogen and water cooling. The majority of the solventwas removed by a rotary evaporator (120° C., 15 mbar). The remainder wassuspended in about 400 g of IPA and centrifuged for 30 min at 4000 rpm(Heraeus, Megafuge 1.0). This procedure was repeated four times. Themodified SWNTs were dried in a vacuum oven for 48 h at 160° C. (10⁻¹mbar). The modified SWNTs were analyzed using XPS, giving the atomiccomposition of the modified SWNTs. The following atomic weight percents(n %) were found.

nitrogen carbon oxygen content Sample content (n %) content (n %) (n %)Purified SWNTs 94.1 4.7 0.6 SWNT modified with APMI 87.4 9.0 3.2 SWNTmodified with APCI 91.2 6.1 2.0

Using these values in combination with the molecular structure of APCIand APMI, the degree of functionalization based on nitrogen can becalculated. The following functionalization values were found.

functionalization value based Sample on nitrogen (mmole/g) SWNT modifiedwith APMI 1.10 SWNT modified with APCI 0.61

EXAMPLE 3

The spinning solution of example 1 was spun on a RandCastle spinningmachine (a small scale spinning machine, Microtruder™ RCP-0250) whichwas adapted to the PPTA process in order to survive the contact withsulfuric acid.

The RandCastle spinning machine consists of the following parts:

-   -   i) Hopper    -   ii) Extruder (diameter=6 mm and length=240 mm; only 150 mm was        used to transport and melt the spinning solution).    -   iii) Four heating units for heating the extruder screw.    -   iv) Spinneret (stainless steel filters: 120; 325; 325; 120 mesh,        6 spinning holes, diameter=80 μm and L/d=0.2).

The following experiment was performed with the carbon nanotubecontaining spinning solution (see Table 1).

TABLE 1 Process settings during spinning Speed Extrusion Extrusion afterDraw Extruder Filament rate rate DR ratio speed diameter Sample (g/min)(m/min) (m/min) (DR) (rpm) (μm) 1 0.77 14.59 74 5.1 40 17.4

In the experiment an air-gap of 0.5 cm was maintained.

The coagulation medium (water) was refreshed with a water flow of 80 to350 ml per minute. The water temperature was approximately 21 to 24° C.

The yarn was wound on a bobbin. Subsequently, the yarn was neutralized,washed, and dried. The following procedure was used:

-   -   The yarn wound on a bobbin was washed in water (slowly streaming        water) for approximately 60 minutes.    -   Subsequently, hydrogen carbonate was added to the water in order        to neutralize the yarn (duration was approximately 1 day) and        the yarn was washed with water (duration was approximately 1        day).    -   The yarn was dried in air for approximately 1 night.

The following fiber properties were obtained.

LD BT TS EAB CMA [dtex] [mN/tex] [GPa] [%] [GPa] 3.2 1196 1.72 2.3 71 LD= linear density BT = breaking tenacity TS = tensile strength EAB =elongation at break CMA = modulus

The mechanical testing of single filaments of a yarn was carried out asfollows.

The filaments were conditioned at 21±1° C. and 65±2% relative humidity(ASTM D1776-98). The linear density of each filament was determinedaccording to ASTM D1577-96 (Option C—Vibroscope) over a length of 20 mm.The tensile tests were carried out on an Instron 5543 tensile testerusing Instron 2712-001 filament clamps with an Arnitel® EL-550 coating.The gage length was set to 100 mm. A pre-tension of 20 mN/tex was used.The clamp speed used was 10 mm/min. The linear density and mechanicalproperties are average values of ten single filaments. The mechanicalproperties of the filaments were determined according to ASTM D885-98.

COMPARISON EXAMPLE 4

According to the prior art method as disclosed in EP 1336673 and using aplunger spinning machine.

A liquid spinning solution (dope) was produced in a Drais mixer of 6liter. During all dope preparation steps, nitrogen was purged into theDrais mixer. The mixing chamber of the Drais mixer is a double-walledchamber. The following procedure was used to prepare the spinningsolution.

-   -   Adding 2022 grams of sulfuric acid (99.8% by weight) to a        preheated mixing chamber (wall temperature=90° C.) while purging        the system with nitrogen.    -   Heating the mixture to a temperature of 90° C.    -   Adding 5 grams of single walled carbon nanotubes. The SWNTs were        dried at 50° C. under vacuum for 8-10 hours.    -   Mixing the solution (liquid sulfuric acid and the SWNTs) (mixing        speed=18 rpm) for 60 minutes at a temperature of 90° C.    -   Adding 494 g of PPTA (having a relative viscosity 5.1) to        mixture and mixing for 3 hours at a wall temperature of 90° C.

The spinning solution was spun on a plunger spinning machine (a smallscale spinning machine) which was adapted to the PPTA process in orderto survive the contact with sulfuric acid. The spinning machine consistsof the following parts:

-   -   i) Heated reservoir (160 ml) for spinning solution        (temperature=87° C.)    -   ii) Injection plunger for transporting spinning solution through        the spinneret.    -   iii) Spinneret (stainless steel filters: 120; 325; 325; 120        mesh, 10 spinning holes, diameter=85 μm and temperature=87° C.).    -   iv)

The following spinning experiments were performed with the carbonnanotube containing spinning solution: extrusion rate=33.6, 50.4, and67.2 m/min.

In none of these experiments it was possible to spin filaments from thecarbon nanotube containing spinning solution and clogging at the surfaceof the spinneret was observed.

COMPARISON EXAMPLE 5

According to the prior art method as disclosed in EP 1336673 and using aRandCastle spinning machine (a small scale spinning machine;Microtruder™ RCP-0250).

A liquid spinning solution (dope) was produced in an IKA kneader of 0.6liter. During all dope preparation steps, nitrogen was purged into theIKA kneader. The mixing chamber of the IKA kneader is a double-walledchamber. The following procedure was used to prepare the spinningsolution.

-   -   Adding 200 grams of sulfuric acid (99.8% by weight) to a        preheated mixing chamber (wall temperature=80° C.) while purging        the system with nitrogen.    -   Heating the mixture to a temperature of 80° C.    -   Adding 0.494 grams of single walled carbon nanotubes. The SWNTs        were dried at 80° C. under vacuum for 8-10 hours.    -   Mixing the solution (liquid sulfuric acid and the SWNTs) (mixing        speed=30 rpm) for 60 minutes at a temperature of 80° C.    -   Adding 48.88 g of PPTA (having a relative viscosity 5.1) to        mixture and mixing for 2.5 hour at a wall temperature of 80° C.

The spinning solution was spun on a RandCastle spinning machine (a smallscale spinning machine as mentioned in Example 3) which was adapted tothe PPTA process in order to survive the contact with sulfuric acid.

The liquid spinning solution was transferred from the kneader to thepre-heated hopper (87° C.) of the spinning machine. The spinningsolution was heated for 2.5 hours in the hopper of the RandCastlespinning machine. The following spinning experiments were performed withthe carbon nanotube containing spinning solution: screw speed=27, 40,50, and 60 rpm.

It was not possible to spin filaments from the carbon nanotubecontaining spinning solution. No transportation through the extruderscrew was possible.

1. A fiber having filaments comprising a mixture of PPTA and nanotubes,wherein the fiber is a multifilament fiber comprising at least 5filaments and the nanotubes have an aspect ratio of at least 100 and across-sectional diameter of 5 nm or less.